The objectives of this Phase I, dose-escalating, long-term tolerance study are to evaluate the pharmacokinetics of oral ganciclovir in HIV-infected infants, children and adolescents, to determine its maximum tolerated dose in these subjects, and to evaluate its long-term safety and tolerance in them. It will also evaluate the effect of oral ganciclovir on the virologic parameters of cytomegalovirus (CMV), i.e., viral load, shedding and resistance to ganciclovir.